1. Field of the Disclosure
This disclosure generally relates to a light control circuit and, more particularly, to a control circuit of a light source in an optical encoder system and an optical encoder system using the same.
2. Description of the Related Art
In an optical detection system, it is generally required to detect a stable light intensity. One method to achieve this requirement is to control a system light source to maintain stable emission intensity.
For example, FIG. 1 shows a conventional optical detection system which includes a light detector 91, a reference voltage generator 93, an error amplifier 95, an NMOS driver 97 and a light emitting diode (LED). The light detector 91 is used to detect modulated light to generate a detected signal, e.g., as shown in FIG. 2A. The light detector 91 also retrieves a common mode voltage VCM of the detected signal as an output signal Vdet. The reference voltage generator 93 outputs a reference voltage Vref based on a desired common mode voltage. The error amplifier 95 compares the output signal Vdet and the reference voltage Vref to cancel common mode noise. The NMOS driver 97 regulates a drive current of the LED according to an output of the error amplifier 95 to control emission intensity thereof.
However, as the system is not perfect and can have a dark current leakage and a reflected light leakage, the common mode voltage VCM of the output signal Vdet can have a dc shift, e.g., FIG. 2B showing that the common mode voltage VCM shifts from 1.15 volt. to 1.17 volt. However, the reference voltage Vref outputted by the reference voltage generator 93 does not change at the same time such that the LED is not controlled to emit light at desired intensity.
Accordingly, it is necessary to provide a light control circuit that can eliminate the dc offset of photodiode current.